Flux and solder paste

ABSTRACT

A soldering flux of the present invention contains, as a base resin, an acrylic resin (A) having an acid value of 0 to 70, and an acrylic resin (B) having an acid value of 30 to 230. The acrylic resin (A) is obtained by polymerization of a monomer mixture containing alkyl (meth)acrylate having an alkyl group having carbon atoms of 12 to 23. The acrylic resin (B) is obtained by polymerization of a monomer mixture containing alkyl (meth)acrylate having an alkyl group having carbon atoms of 6 to 10. The acid value of the acrylic resin (B) is higher than the acid value of the acrylic resin (A), and a difference between these two resins is 15 or more.

TECHNICAL FIELD

The present invention relates to a soldering flux for use in solderjoining of circuit components or the like onto a circuit board, such asa printed circuit board of an electronic device, as well as a solderpaste composition using the soldering flux.

BACKGROUND ART

Various kinds of soldering fluxes and solder paste compositions haveconventionally been used for solder joining of electronic circuitcomponents, or the like. Particularly, the flux removes metal oxide onsolder and on a substrate surface, and also prevents reoxidation ofmetal during soldering. Hence, the flux reduces surface tension of thesolder and is therefore indispensable to satisfactorily perform thesoldering.

However, the conventional flux and solder paste composition may causethe problem that cracks occur in a flux residue after the soldering andwater enters the cracks, thus producing a short-circuit defect betweencomponent leads. There is a high probability that this problem occursparticularly on an on-vehicle substrate subjected to a large temperaturedifference and large vibration during use.

The method for applying solder paste can be roughly classified intoprinting method and discharge method.

The printing method is one in which a metal mask, a silk screen, or thelike, each having holes disposed in soldering portions, is mounted on aprinted circuit board, and solder paste is applied from thereabove. Thedischarge method is one in which solder paste is applied to thesoldering portions one by one by using a dispenser or the like. Therearises a disadvantage that the application is not performable on afine-pitch pattern by the discharge method. For example, the printingmethod is employed in solder joining of electronic circuit components orthe like to a fine-pitch circuit board.

Mounting technology is highly densified with downsizing electronicdevices, and there is an increasing trend towards a finer pitch.Therefore, the solder paste is required to have excellent printability(transfer performance) in addition to conventionally requiredcharacteristics (stability, reliability, and the like). For example,when using the metal mask, the printability is to efficiently transferto the substrate the solder paste adhered to a wall surface or the likeof an opening in the metal mask. Several means, such as miniaturizationof metal particle size, and increasing the amount of wax, have beenproposed so far in order to improve the printability. Although theminiaturization of metal particle size ensures improving theprintability, “storage stability” and “wettability” are poor. Theincreasing the amount of wax makes it difficult to adjust viscosity andthe wettability is apt to be poor.

The present applicant has found so far that a thermoplastic acrylicresin having glass transfer temperature of less than −50° C. is used asa base resin in order to suppress cracks of the flux residue, and thatan active action can be promoted when the acid value of thethermoplastic acrylic resin is 50 mg KOH/g or more (Patent Document 1).

Patent Document 2 describes that a resin having acid value of 50 mgKOH/g or less is added to solder paste in order to improve washabilityagainst the flux residue after soldering, that the resin is rosinobtained by purifying a natural resinpine tree gum, and that a resinhaving a high acid value is mixed in order to compensate for the activeaction of the resin having a low acid value. However, there is nodescription of the necessity for improving the printability, and thereis no description of the use of a resin other than the rosin. The fluxresidue is to be removed by washing, and hence it is difficult toimprove the crack resistance of the flux residue.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Publication No. 2008-62252

Patent Document 2: Japanese Unexamined Patent Publication No. H5-212584

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a soldering flux thatexerts excellent effect in terms of all of the printability andwettability of solder paste, and the crack resistance of the fluxresidue, as well as a solder paste composition using the soldering flux.

Means for Solving the Problems

The present inventor has conducted intensive research to solve theforegoing problems and has completed the present invention by findingsolutions made of the following configurations.

(1) A soldering flux contains, as a base resin, an acrylic resin (A)having an acid value of 0 to 70, and an acrylic resin (B) having an acidvalue of 30 to 230. The acrylic resin (A) is obtained by polymerizationof a monomer mixture containing alkyl (meth)acrylate having an alkylgroup having carbon atoms of 12 to 23. The acrylic resin (B) is obtainedby polymerization of a monomer mixture containing alkyl (meth)acrylatehaving an alkyl group having carbon atoms of 6 to 10. The acid value ofthe acrylic resin (B) is higher than the acid value of the acrylic resin(A), and a difference between the acid values of these two resins is 15or more.

(2) In the soldering flux as described in the above (1), a content ofthe acrylic resin (A) is 10 to 30% by mass in a total amount of flux,and a content of the acrylic resin (B) is 10 to 30% by mass in the totalamount of flux.

(3) In the soldering flux as described in the above (1) or (2), theacrylic resin (A) is an acrylic resin obtained by polymerization of amonomer mixture containing at least 50% by mass of alkyl (meth)acrylatehaving an alkyl group having carbon atoms of 12 to 23.

(4) In the soldering flux as described in any one of the above (1) to(3), the acrylic resin (B) is an acrylic resin obtained bypolymerization of a monomer mixture containing at least 50% by mass ofalkyl (meth)acrylate having an alkyl group having carbon atoms of 6 to10.

(5) In the soldering flux as described in any one of the above (1) to(4), the acrylic resin (A) has a weight average molecular weight of30000 or less.

(6) In the soldering flux as described in any one of the above (1) to(5), the acrylic resin (B) has a weight average molecular weight of30000 or less.

(7) A solder paste composition contains the soldering flux as describedin any one of the above (1) to (6), and solder alloy powder.

Effect of the Invention

The soldering flux of the present invention exerts excellent effect interms of all of the printability and wettability of the solder paste,and the crack resistance of the flux residue.

Preferred Embodiments for Carrying Out the Invention (Soldering Flux)

A soldering flux of the present invention (hereinafter generallyreferred to simply as “flux”) contains, as a base resin, an acrylicresin (A) and an acrylic resin (B) having different acid values.

The acrylic resins (A) and (B) are respectively polymers obtained bypolymerization of a monomer mixture containing alkyl (meth)acrylate. Inthe present specification, “(meth)acrylate” implies acrylate ormethacrylate.

The acrylic resin (A) is capable of exerting the effect of the presentinvention as long as it is obtained by polymerization of a monomermixture containing at least 50% by mass of alkyl (meth)acrylate havingan alkyl group having carbon atoms of 12 to 23. The acrylic resin (B) iscapable of exerting the effect of the present invention as long as it isobtained by polymerization of a monomer mixture containing at least 50%by mass of alkyl (meth)acrylate having an alkyl group having carbonatoms of 6 to 10.

Examples of the alkyl (meth)acrylate having the alkyl group having thecarbon atoms of 12 to 23 include 2,2-dimethyllauryl (meth)acrylate,2,3-dimethyllauryl (meth)acrylate, 2,2-dimethylstearyl (meth)acrylate,2,3-dimethylstearyl (meth)acrylate, isolauryl (meth)acrylate,isomyristyl (meth) acrylate, isostearyl (meth) acrylate, and isobehenyl(meth)acrylate. These alkyl (meth)acrylates may be used alone, or two ormore kinds of these may be used together. Among others, the alkyl(meth)acrylate having the alkyl group having the carbon atoms of 12 to20 is preferred.

Examples of the alkyl (meth)acrylate having the alkyl group having thecarbon atoms of 6 to 10 include hexyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, octyl (meth)acrylate, and nonyl (meth)acrylate. Thesealkyl (meth)acrylates may be used alone, or two or more kinds of thesemay be used together.

Besides the above alkyl (meth)acrylates, other monomers that arecopolymerizable with these alkyl (meth)acrylates may be used. Examplesof these other monomers include non-ionic monomers (for example, acrylamide, styrene, α-olefin, alkyl (meth)acrylates having an alkyl grouphaving carbon atoms of 1 to 5 (methyl (meth)acrylate, ethyl(meth)acrylate, hydroxyethyl (meth)acrylate, and butyl (meth)acrylate)),and anionic monomers (for example, acrylic acid, methacrylic acid, anditaconic acid).

The acid value of the acrylic resin is determined by a content of anorganic acid monomer (for example, acrylic acid, methacrylic acid, etc.)in a monomer ingredient. That is, when the monomer ingredient containsonly the alkyl (meth)acrylate and the nonionic monomer, an obtainedacrylic resin has an acid value of 0 mg KOH/g, and the acid valueincreases with increasing the content of the organic acid monomer.

The present invention has a characteristic feature that the acid valueof the acrylic resin (B) is higher than the acid value of the acrylicresin (A) and a difference between these two resins is 15 or more. Theacrylic resin (A) has an acid value of 0 to 70 mg KOH/g, preferably 20to 50 mg KOH/g. The acrylic resin (B) has an acid value of 30 to 230 mgKOH/g, preferably 60 to 150 mg KOH/g.

The method for synthesizing the acrylic resins (A) and (B) is notparticularly limited, and a well-known method may be employed. Forexample, the monomer ingredient constituting the acrylic resins (A) and(B) may be subjected to radical polymerization using solvent,polymerization initiator, chain transfer agent, and the like asnecessary. The polymerization initiator used for the radicalpolymerization is not particularly limited as long as it is a compoundthat decomposes to generate radicals. Examples thereof include azo typeinitiators and peroxide type initiators. Among others, azo typeinitiators, such as azobisisobutyronitrile (AIBN),azobismethylbutyronitrile (ABNE), and azobisdimethylvaleronitrile (ABNV)are preferred.

The acrylic resins (A) and (B) preferably have a weight averagemolecular weight of 30000 or less, more preferably 5000 to 30000. Owingto the acrylic resins (A) and (B) having the weight average molecularweight of 30000 or less, wettability is further improved and theiradhesion to a squeegee is far less likely to occur. It is, of course,possible to use the acrylic resins (A) and (B) having different weightaverage molecular weights.

The weight average molecular weight is usually measured with a gelpermeation chromatography (GPC).

A content of acrylic resin (A) is 10 to 30% by mass, preferably 10 to25% by mass in a total amount of flux. A content of acrylic resin (B) isalso 10 to 30% by mass, preferably 10 to 25% by mass in the total amountof flux. The printability and wettability of the solder paste and thecrack resistance of the flux residue can be further improved by usingthe acrylic resins (A) and (B) in the above-mentioned range.

The flux of the present invention usually contains an activator, athixotropic agent, an organic solvent, and the like. Other base resinmay further be incorporated in such a range as not to impair the effectof the present invention.

Examples of the other base resin include styrene-maleic acid resin,epoxy resin, urethane resin, polyester resin, phenoxy resin, terpeneresin, and rosin-based resins. Examples of the rosin include gum rosin,tall rosin, wood rosin, and derivatives of these rosins. Examples ofthese derivatives include polymerized rosin, acrylated rosin,hydrogenated rosin, disproportionated rosin, formylated rosin, rosinester, rosin modified maleic acid resin, rosin modified phenol resin,and rosin modified alkyd resin.

Examples of the activator include amines (diphenylguanidine,naphthylamine, diphenylamine, triethanolamine, monoethanolamine, etc.),amine salts (polyamines such as ethylenediamine, organic acid salts ofamine such as cyclohexylamine, ethylamine and diethylamine, andinorganic acid (mineral acids such as hydrochloric acid, and sulfuricacid) salts, organic acids (dicarboxylic acids such as succinic acid,adipic acid, glutaric acid, sebacic acid, and maleic acid; fatty acidssuch as myristic acid, palmitic acid, stearic acid, and oleic acid;hydroxy carboxylic acids such as lactic acid, dimethylolpropionic acid,and malic acid; benzoic acid, phthalic acid, trimellitic acid), aminoacids (glycine, alanine, aspartic acid, glutamic acid, valine, etc.),and hydrohalogenic acid salts of aniline (aniline hydrobrominate, etc.).

Examples of the thixotropic agent include hydrogenated castor oil,beeswax, carnauba wax, stearic acid amide, and hydroxystearic acidethylene bisamide.

Examples of the organic solvent include alcohol type solvents (ethylalcohol, isopropyl alcohol, ethyl cellosolve, butyl carbitol, hexylcarbitol, etc.), ester type solvents (ethyl acetate, butyl acetate,etc.), and hydrocarbon type solvents (toluene, turpentine, etc.). Amongothers, isopropyl alcohol, butyl carbitol, and hexyl carbitol arepreferred. Isopropyl alcohol has excellent volatility and solubility inthe activator, and is suitably used for liquid flux or the like.Meanwhile, when the flux is mixed with solder alloy powder so as to beused as a solder paste composition, polyhydric alcohol ethers having ahigh boiling point, such as butyl carbitol and hexyl carbitol, aresuitably used.

In the flux of the present invention, the contents of the acrylic resin(A), the acrylic resin (B), the other base resin, the activator, thethixotropic agent, and the organic solvent in the total amount of theflux are as follows.

Acrylic resin (A): 10 to 30% by mass, preferably 10 to 25% by mass;

Acrylic resin (B): 10 to 30% by mass, preferably 10 to 25% by mass;

Other base resin: 0 to 20% by mass, preferably 5 to 15% by mass;

Activator: 0 to 30% by mass, preferably 5 to 25% by mass;

Thixotropic agent: 0 to 13% by mass, preferably 1 to 8% by mass; and

Organic solvent: 0 to 35% by mass, preferably 10 to 30% by mass

Furthermore, the flux of the present invention may contain additives,such as an antioxidant, a chelating agent, an anti-rust agent, asnecessary in such a range as not to impair the effect of the presentinvention. These additives may be added, for example, when mixing theflux and the solder alloy powder together.

(Solder Paste Composition)

The solder paste composition of the present invention contains the fluxof the present invention and the solder alloy powder. The solder alloypowder is not particularly limited. Examples thereof include Sn-Pballoy, alloy obtained by adding silver, bismuth, and indium to the Sn—Pballoy, Sn—Ag type alloys, Sn—Cu type alloys, and Sn—Ag—Cu type alloys.In consideration of environmental impact, lead-free alloys, such asSn—Ag type alloys, Sn—Cu type alloys, and Sn—Ag—Cu type alloys, arepreferred. The mean particle size of the solder alloy powder is notparticularly limited, but is preferably, for example, approximately 10to 40 tim.

A mass ratio of the flux and the solder alloy powder (the flux : thesolder alloy powder) is not particularly limited, but may be suitablyset according to the intended use of the solder paste, or the like. Forexample, the mass ratio of approximately 8:92 to 15:85 is preferred.

The solder paste composition of the present invention is to be appliedonto a substrate by the dispenser or screen printing when solder joiningelectronic device components or the like. After the application,preheating is conducted at approximately 150 to 200° C., and reflow isconducted at a maximum temperature of approximately 170 to 250° C. Theapplication onto the substrate and the reflow may be conducted in theatmosphere or an inert gas atmosphere of nitrogen, argon, helium, or thelike.

EXAMPLES

The present invention is specifically described below with reference toexamples and comparative examples, but it should be construed that thepresent invention is in no way limited to the following examples.

<Synthesis of Acrylic Resin> Synthesis Example 1

As a reaction solvent, 30 parts by mass of hexyl carbitol was loadedinto a reaction vessel (glass flask) including a thermometer and anitrogen introduction tube. This was heated to 120° C. while beingstirred in a nitrogen atmosphere. Subsequently, a monomer solution wasprepared by mixing 65 parts by mass of stearyl methacrylate as a monomeringredient and 5 parts by mass of AIBN as a polymerization initiator.The monomer solution was dropped in the reaction vessel for two hoursand was allowed to react at 120° C. After all the monomer solution wasdropped, this was aged at 120° C. for two hours, thereby obtaining anacrylic resin (A1) (acid value: 0 mg KOH/g, weight average molecularweight (Mw): 10000, and glass transition temperature (Tg): 27° C.). Theacid value and the weight average molecular weight were respectivelymeasured with the following methods.

<Acid Value Measurement>

An acid value measurement was conducted according to acid valueneutralization titration method of JIS 0070. A 1 g of resin as a samplewas loaded into a conical flask, and 100 mL of solvent (a mixed solutionof toluene and ethanol, and a mixing ratio of 1:1) and several drops ofphenolphthalein solution (indicator) were added thereto so as tocompletely dissolve the sample. Thereafter, titration was conducted with0.1 mol/L of potassium hydroxide ethanolic solution, and a time pointwhen a light pink of the indicator was colored for 30 seconds was takenas a terminal point. The acid value was obtained using the followingequation.

Acid value=(A×f×5.611)/S

(wherein A is a titration quantity (mL), f is a factor of potassiumhydroxide ethanolic solution, and S is a mass (g) of the sample.)

<Measurement of Weight Average Molecular Weight>

A molecular weight distribution of the resin was measured by gelpermeation chromatography (GPC), and a weight average molecular weightwas obtained.

Synthesis Examples 2-14 and Comparative Synthesis Examples 1-4

Acrylic resins (A2) to (A6), acrylic resins (B1) to (B8), and acrylicresins 1 to 4 were obtained in the same procedure as in SynthesisExample 1, except that ingredients illustrated in Table 1 were used intheir respective proportions illustrated in Table 1. The acid value, Mw,and Tg of each of the obtained acrylic resins are illustrated in Table1.

The acrylic resins (A1) to (A6) obtained in Synthesis Examples 1 to 6are included in the acrylic resin (A), and the acrylic resins (B1) to(B8) obtained in Synthesis Examples 7 to 14 are included in the acrylicresin (B).

TABLE 1 Polymerization Monomer ingredient initiator Tg Acid value Resin(parts by mass) (parts by mass) Mw (° C.) (mgKOH/g) Synthesis A1Methacrylate Methacrylic acid AIBN 12000 28 0 Example 1 (70) (0) (5)Synthesis A2 Methacrylate Methacrylic acid AIBN 12000 28 10 Example 2(69) (1) (5) Synthesis A3 Methacrylate Methacrylic acid AIBN 12000 31 30Example 3 (67) (3) (5) Synthesis A4 Methacrylate Methacrylic acid AIBN12000 32 42 Example 4 (65.8) (4.2) (5) Synthesis A5 MethacrylateMethacrylic acid AIBN 12000 33 50 Example 5 (64.8) (5.2) (5) SynthesisA6 Methacrylate Methacrylic acid AIBN 12000 36 70 Example 6 (62.8) (7.8)(5) Synthesis B1 Acrylate Methacrylic acid Benzoate 8000 −80 30 Example7 (97) (3) (5) Synthesis B2 Acrylate Methacrylic acid Benzoate 8000 −7940 Example 8 (96) (4) (5) Synthesis B3 Acrylate Methacrylic acidBenzoate 8000 −77 50 Example 9 (94.8) (5.2) (5) Synthesis B4 AcrylateMethacrylic acid Benzoate 8000 −74 64 Example 10 (93.6) (6.4) (5)Synthesis B5 Acrylate Methacrylic acid Benzoate 8000 −73 70 Example 11(92.8) (7.2) (5) Synthesis B6 Acrylate Methacrylic acid Benzoate 8000−70 90 Example 12 (90.8) (9.2) (R) Synthesis B7 Acrylate Methacrylicacid Benzoate 8000 −53 170 Example 13 (82.7) (17.3) (5) Synthesis B8Acrylate Methacrylic acid Benzoate 8000 −42 230 Example 14 (77.2) (22.8)(5) Comparative 1 Methacrylate Methacrylic acid AIBN 12000 37 80Synthesis (62) (8) (5) Example 1 Comparative 2 Acrylate Methacrylic acidBenzoate 8000 −83 10 Synthesis (9 9) (1) (5) Example 2 Comparative 3Acrylate Methacrylic acid Benzoate 8000 −82 20 Synthesis (98) (2) (5)Example 3 Comparative 4 Acrylate Methacrylic acid Benzoate 8000 −39 240Synthesis (76) (24) (5) Example 4 “Methacrylate” of the monomeringredient is stearyl methacrylate. “Acrylate” of the monomer ingredientis 2-ethylhexyl acrylate. “Benzoate” of the polymerization initiator ist-butyl peroxybenzoate. 30 parts by mass of hexyl carbitol is used as asolvent in all Synthesis Examples.

<Preparation of Flux and Solder Paste Composition>

Each of Examples and Comparative Examples employed tributylaminehydrobromide (denoted by “HBr” in Tables 2 and 3) and adipic acid as theactivator, castor wax (denoted by “wax” in Tables 2 and 3) as thethixotropic agent, and hexyl carbitol (denoted by “carbitol” in Tables 2and 3) as the organic solvent.

Example 1

As illustrated in Table 2, 20% by mass of the acrylic resin (A1), 20% bymass of the acrylic resin (B1), 8% by mass of adipic acid, 0.5% by massof tributylamine hydrobromide, 30% by mass of hexyl carbitol, and 6.5%by mass of castor wax were loaded in a vessel. The vessel was thenheated to dissolve and mix these ingredients, and thereafter, the vesselwas cooled to obtain flux. The resultant flux and solder alloy powder(Sn-3.0Ag-0.5Cu) were mixed together in the ratio of the flux to thesolder metal powder, 11:89 (mass ratio), thereby obtaining a solderpaste composition.

Examples 2-23 and Comparative Examples 1-15

Individual fluxes were obtained in the same procedure as in Example 1,except that ingredients illustrated in Tables 2 and 3 were used in theirrespective proportions illustrated in Tables 2 and 3. Subsequently,individual solder paste compositions were respectively obtained in thesame procedure as in Example 1, except for respectively using theresultant fluxes.

The solder paste compositions obtained in these Examples and ComparativeExamples were used to evaluate in terms of (1) printability and (2)wettability of the solder paste, and (3) crack resistance of the flaxresidue with the following methods.

(1) Printability of Solder Paste

Using a substrate for evaluating printability (a glass epoxy substratehaving a 0.5 mm pitch of 10×10 pins and a 0.25 mmϕ opening pattern ofBGA (ball grid array), continuous printability for 20 pieces wasevaluated according to the following criteria by using a mask having thecorresponding thickness of 150 μm.

Symbol “O” denotes the case where less than 20 percent of the 10×10 pinswere fractured on all substrates; and

Symbol “×” denotes the case where 20 percent or more of the 10×10 pinswere fractured on all substrates.

(2) Wettability

The solder paste composition was printed on a substrate having thereon aQFP (quad flat package) pattern of 0.8 mm pitch by using a metal maskhaving the same pattern and a thickness of 200 μm. Preheating at 175±5°C. for 80±5 seconds in the atmosphere was carried out within 10 minutesafter the printing, followed by reflow at a maximum temperature of235±5° C. A solder ball occurrence situation serving as an index wasobserved by counting the number of solder balls occurred around 80 pads(80 soldering portions) with a stereoscopic microscope at ×20magnification. It was evaluated as having excellent wettability when thenumber of solder balls was 10 or less.

Symbol “O” denotes the case where the number of solder balls is 10 orless; and

Symbol “×” denotes the case where the number of solder balls exceeds 10.

(3) Crack Resistance of Flux Residue

The substrate after subjected to the above wettability test was used asa test piece. The test piece was subjected to a cooling/heating cycleload under conditions of 1000 cycles with the range from −40° C.×30minutes to 125° CX30 minutes as one cycle. Thereafter, residue crackoccurrence states at the soldering portions on the substrate werevisually observed and evaluated according to the following criteria.

Symbol “O” denotes the case where no crack was observed; and

Symbol “×” denotes the case where cracks were observed.

TABLE 2 Base resin Thixotropic Organic Resin (A) Resin (B) Activatoragent solvent Crack (% by mass) (% by mass) (% by mass) (% by mass) (%by mass) Printability Wettability Resistance Examples 1 Al B1 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 2 A1 B2 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 3 A1 B3 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 4 A1 B6 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 5 A1 B7 Adipicacid (8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 6 A1 B8 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 7 A2 B1 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 8 A2 B2 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 9 A2 B3 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 10 A3 B3 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 11 A3 B5 Adipicacid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) H6r(0.5) (6.5) (30) 12 A3 B6 Adipicacid (8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 13 A3 B7Adipic acid (8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 14 A3B8 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 15 A4B4 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 16 A4B6 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 17 A5B5 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 18 A5B6 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 19 A5B7 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 20 A5B8 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 21 A6B6 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 22 A6B7 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 23 A6B8 Adipic acid(8) Wax Carbitol ∘ ∘ ∘ (20) (20) HBr(0.5) (6.5) (30) 15%by mass of gum rosin is further included in the base resin all Examples.“HBr” of the activator is tributylamine hydrobromide. “Wax” of thethixotropic agent is castor wax. “Carbitol” of the organic solvent ishexyl carbitol.

TABLE 3 Base resin Thixotropic Organic Resin (A) Resin (B) Activatoragent solvent Crack (% by mass) (% by mass) (% by mass) (% by mass) (%by mass) Printability Wettability Resistance Comparative 1 A1 Resin 2Adipic acid(8) Wax Carbitol x ∘ x Examples (20) (20) HBr(0.5) (6.5) (30)2 A1 Resin 4 Adipic acid(8) Wax Carbitol ∘ ∘ x (20) (15) HBr(0.5) (6.5)(35) 3 A2 Resin 3 Adipic acid(8) Wax Carbitol x ∘ ∘ (20) (20) HBr(0.5)(6.5) (30) 4 A3 B2 Adipic acid(8) Wax Carbitol x ∘ ∘ (20) (20) HBr(0.5)(6.5) (30) 5 A3 Resin 4 Adipic acid(8) Wax Carbitol ∘ ∘ x (20) (15)HBr(0.5) (6.5) (35) 6 A5 B3 Adipic acid(8) Wax Carbitol x ∘ ∘ (20) (20)HBr(0.5) (6.5) (30) 7 A5 Resin 4 Adipic acid(8) Wax Carbitol ∘ ∘ x (20)(15) HBr(0.5) (6.5) (35) 8 A6 B5 Adipic acid(8) Wax Carbitol x ∘ ∘ (20)(20) HBr(0.5) (6.5) (30) 9 A6 Resin 4 Adipic acid(8) Wax Carbitol ∘ ∘ x(20) (10) HBr(0.5) (6.5) (40) 10 Resin 1 B6 Adipic acid(8) Wax Carbitolx ∘ x (20) (20) HBr(0.5) (6.5) (30) 11 Resin 1 B7 Adipic acid(8) WaxCarbitol ∘ ∘ x (20) (15) HBr(0.5) (6.5) (35) 12 Resin 1 B8 Adipicacid(8) Wax Carbitol ∘ ∘ x (20) (20) HBr(0.5) (6.5) (30) 13 A1 — Adipicacid(8) Wax Carbitol ∘ ∘ x (40) HBr(0.5) (6.5) (30) 14 — B4 Adipicacid(8) Wax Carbitol x ∘ ∘ (20) HBr(0.5) (6.5) (30) 15 — — Adipicacid(8) Wax Carbitol x x x H5r(0.5) (10) (66.5) 15% by mass of gum rosinis further included in the base resin all Comparative Examples. “HBr” ofthe activator is tributylamine hydrobromide. “Wax” of the thixotropicagent is castor wax. “Carbitol” of the organic solvent is hexylcarbitol. Resins 1 to 4 are acrylic resins whose acid values are outsidethe range.

It can be seen from Table 2 that the solder paste compositions ofExamples 1 to 23 have excellent results in terms of all of (1) theprintability and (2) the wettability of the solder paste, and (3) thecrack resistance of the flux residue. On the other hand, it can be seenfrom Table 3 that the solder paste compositions of Comparative Examples1 to 15 are poor in terms of at least one of (1) the printability and(2) the wettability of the solder paste, and (3) the crack resistance ofthe flux residue.

1. A soldering flux comprising: an acrylic resin (A) as a base resinwhich has an acid value of 0 to 70 and is obtained by polymerization ofa monomer mixture containing alkyl (meth)acrylate having an alkyl grouphaving carbon atoms of 12 to 23; and an acrylic resin (B) as a baseresin which has an acid value of 30 to 230 and is obtained bypolymerization of a monomer mixture containing alkyl (meth)acrylatehaving an alkyl group having carbon atoms of 6 to 10, wherein the acidvalue of the acrylic resin (B) is higher than the acid value of theacrylic resin (A), and a difference between the acid values of these tworesins is 15 or more.
 2. The soldering flux according to claim 1,wherein a content of the acrylic resin (A) is 10 to 30% by mass in atotal amount of flux, and a content of the acrylic resin (B) is 10 to30% by mass in the total amount of flux.
 3. The soldering flux accordingto claim 1, wherein the acrylic resin (A) is an acrylic resin obtainedby polymerization of a monomer mixture containing at least 50% by massof alkyl (meth)acrylate having an alkyl group having carbon atoms of 12to
 23. 4. The soldering flux according to claim 1, wherein the acrylicresin (B) is an acrylic resin obtained by polymerization of a monomermixture containing at least 50% by mass of alkyl (meth)acrylate havingan alkyl group having carbon atoms of 6 to
 10. 5. The soldering fluxaccording to claim 1, wherein the acrylic resin (A) has a weight averagemolecular weight of 30000 or less.
 6. The soldering flux according toclaim 1, wherein the acrylic resin (B) has a weight average molecularweight of 30000 or less.
 7. A solder paste composition comprising thesoldering flux according to claim 1, and solder alloy powder.